Inverse dynamics moments/forces at the pelvis

[Melanie Hook]

Hi everyone,
Could someone please explain the coordinate system for the moments and forces computed by inverse dynamics? I'm trying to understand where the residuals at the pelvis are coming from.

I think the forces (pelvis_tx_force, pelvis_ty_force, and pelvis_tz_force) are expressed in the global coordinate system. Is this correct?

I am also confused about the moments (pelvis_tilt_moment, pelvis_list_moment and pelvis_rotation_moment). Which coordinate frame are these moments expressed in (pelvis? ground?) and about which point are these moments taken?

Thank you for your help!
Melanie Hook
melaniehook@vt.edu

[Quinten Humphrey]

You are correct with the coordinates forces as tx, ty, and tz mean that the pelvis translates along those axes.

As for the moments, those are only applied to the pelvis. In terms of forces and moments applied to the ground-pelvis joint, those will be defined as Fx, Fy, Fz, Mx, My and Mz. Hope this helps explain it and that you have already figured this problem out.

Bests,
Quinten

[Melanie Hook]

Hi Quinten,
Thank you so much for your response! I am still confused by the coordinate system conventions. Could you please explain the difference between the forces/moments at the pelvis vs at the ground-pelvis joint? The variables that OpenSim inverse dynamics have been providing are pelvis_tx_force, pelvis_ty_force, pelvis_tz_force, pelvis_tilt_moment, pelvis_list_moment and pelvis_rotation_moment. If you could also clarify what those variables represent, that would be great!

My overall goal is to predict the ground reaction forces using the procedure in this paper:
Computation of ground force using Zero Moment Point
https://www.sciencedirect.com/science/a ... 5278#bib16
After simulating inverse dynamics without specifying ground reaction forces, the residuals at the pelvis are transformed to recover the center of pressure, GRM, and GRF.

Thank you for your help!
Melanie Hook
melaniehook@vt.edu

[Quinten Humphrey]

I believe the ground-pelvis moments is where the ground reaction forces, from the force plates, are applied since your first parent/child joint needs to be connected to the ground.
pelvis_tx_force, pelvis_ty_force, pelvis_tz_force are the forces on the pelvis relative in the x-, y-, and z-directions when the model is being simulated. Likewise, pelvis_tilt_moment, pelvis_list_moment and pelvis_rotation_moment are the moments for the pelvis about the x-, z, and y-axis.

Hope this has been helpful!